* Close enough to half a letter size, compatibility in a pinch
* A5 binders are nice and portable, Letter binders are backpack stretching monstrosities
* In the digital age paper is often used for smaller amounts of text, letter is less necessary
* Less desk space needed if it's something like a diagram or plan you're working on
* A6 is even more awesome, but most cheap A6 binders are fake and actually some other kinda ugly IMHO aspect ratio
* Fits in smaller bags and boxes
* Easily hold the binder with one hand to write in while standing
* 6 holes has way more redundancy than 3 for when the paper tears
* Less waste if you frequently do stuff that doesn't entirely fill up a Letter size
* If holding an individual sheet, it's got less leverage to flop around and will feel more stiff and solid than letter/A4.
* Most books are closer to that size, and books are like, everyone's favorite thing to do with paper
Why aren't we using this more? Or are we? Have any offices or schools started using it?
For loose sheets, I'd go with B5, 26 holes paper and binder. They are two-third the size of letter, which is a good compromise to have some empty space for writing and drawing, while not that small (like A5) that are a hassle to bind.
That's why we use it.
Letter size is the norm in the US because the paper size system in the US has practical economic advantages over the A/B system used elsewhere.
The economic advantage is that larger sheets can be cut into smaller sheets...letter is half 11x17, 11x17 is half 22x17, and so on.
But it's actually more than that. Because 8.5 x 11 can be finished from 9x12, 11x17 from 12x18...
This means standard size rolls from paper mills.
It means printers can inventory only large sheets and get standard sizes from them.
All of that tends to make printing cheaper and easier and more accessible.
Anyway, if you want a special size paper, a paper cutter is perhaps the simplest thing that might work. It will let you take paper size matters into your own hands, though at the risk of losing a finger tip or thumb.
Good luck.